Systems for detecting the sirens of emergency vehicles to provide a warning signal to the driver of an automobile have been known for many years. In the most simple form, such systems employ a band-pass filter or comparable device to respond to signals only within the frequency range of emergency vehicle sirens. Whenever such a signal is received in such a system, an alarm or warning device, in the form of a light or sound, is operated within the automobile. Such simple systems, however, have not been satisfactory since they are easily triggered by "noise" signals having nothing to do with emergency vehicle sirens. After activation of the alarm device by a number of "false alarms", a driver of an automobile equipped with such a system has a tendency to ignore the alarm when it sounds, thereby defeating the purpose of the system.
In the past, particularly before the advent of automobile air conditioning, automobiles frequently were operated with the windows open. In addition, the radios or sound systems in older automobiles are not of the "sound-surround" type; so that drivers of such automobiles generally were able to hear the siren of an approaching emergency vehicle while it still was some distance away.
In contrast to older automobiles, modern automobiles are extremely well sound proofed. Most such automobiles are operated with the windows closed practically all of the time. In the winter, the heater system of the automobile is used to keep the interior warm; and in the summer, an air conditioning system is used to cool the interior of the vehicle. Whether the heater system is being operated or the air conditioning system is being operated, an accompanying fan, with a relatively high degree of noise, is operating within the vehicle. In addition, most modern automobiles have radios or extensive sound entertainment systems with multiple speakers located at various points in the automobile in them. If the radio or sound system is being played with the window closed, and if at the same time either the heater system or the air conditioner is operating, it is difficult, if not impossible, for the occupants of the automobile to hear the siren of an approaching emergency vehicle until such an emergency vehicle is extremely close to the automobile. In fact, in many cases the driver and other occupants of an automobile do not hear the siren of an approaching emergency vehicle until such emergency vehicle is only a few feet away. Consequently, a high potential for an accident between the automobile and the emergency vehicle exists.
Emergency vehicle sirens typically operate in one or the other of three different operating modes, such as "yelp", "wail", or "hi-low". Whichever operating mode is used, the typical frequency range for the sirens in any of the modes extends from a lower frequency of approximately 500 Hz to a upper frequency of approximately 1600 Hz. For a "yelp" siren, the frequency variation is accomplished by a sweep upwardly from the low end of this frequency range to the upper end, with a more sudden drop back down again to the low frequency, for each single cycle of the "yelp" mode. This cycle then is repeated at a rate of 1 to 4 cycles per second. The exact frequency range covered and the cycle repetition rate varies somewhat for different models and types of siren, but the variation for both the frequency range and the repetition rate is within the range mentioned here.
For a "wail", the frequency range again is from a low frequency of approximately of 500 Hz, changing substantially in accordance with a sine-wave pattern, to an upper frequency (approximately 1600 Hz) and back down again on a continuous basis. The repetition rate or frequency of this sine-wave variation generally is at a lower repetition frequency than for the "yelp" operating mode.
"Hi-low" sirens usually employ two frequencies, with a sudden transition between the low frequency to the high frequency and back again at a rate of repetition similar to the repetition rate used in a "wail" siren.
To minimize false operation of an alarm, more sophisticated tone detector systems than the simple band-pass detection systems mentioned above are necessary. Seven (7) patents which are directed to types of pattern recognition for emergency vehicle sirens are the Helliker U.S. Pat. No. 3,735,342; Koehler U.S. Pat. No. 3,859,623; Stefanov U.S. Pat. No. 4,158,190; Jensen U.S. Pat. No. 4,625,206; Nelson U.S. Pat. No. 4,806,931; and two Patents to Bernstein et al. No. 4,785,474 and No. 4,759,069. All of these patents are directed to systems which recognize specific frequencies within the "siren" band of signals typically produced by emergency vehicles. In addition, all of these systems attempt to minimize false triggering of the alarm circuit by noise signals.
The system disclosed in the Helliker Patent provides pattern recognition by cascading the detected outputs of tuned filters with one another, so that an output signal is obtained only when the tuned filters all produce an output within a pre-established interval. The system continuously is reset (thus establishing the pre-established time interval); so that even if noise signals should activate one or more of the tuned filter circuits, no output is obtained. It also is necessary for all of the signals to occur in the proper sequence or no output is obtained. If, as a result of interference, an improperly operating siren, or an aberration in the siren frequency (such as caused by sound bouncing off of the walls of buildings or the like), the system of Helliker may be reset without activating the alarm, even in the presence of a siren signal which it is desired to detect. In addition, the Helliker system requires multiple tuned filters which are bulky and expensive.
The Jensen and Nelson Patents are not directed to alarms for use within an automobile, but the systems of these patents are concerned with the actuation of a traffic signal light control to cause the signal light to be operated in response to the detected siren sound pattern. Typically, the signal lights are caused to be red in one direction and green in another, as determined by the direction of the source of the siren sound pattern detected.
In the Jensen Patent, a plurality of tuned narrow band band-pass filters must be operated in the proper sequence to provide the frequency detection representative of a siren. Detection of one frequency enables the circuit to detect signals of the next higher (or lower) frequency, and so on. Out-of-sequence tones disable the circuit and reset it. In addition, the Jensen system includes provisions for determining the direction from which the emergency vehicle siren in approaching.
The Nelson Patent also is directed to a directional control system for operating the traffic signal lights at an intersection to cause red lights to appear in all directions except the one from which the emergency vehicle is approaching. Nelson, however, employs a microprocessor programmed with specific pattern recognition algorithms to continuously sample the received input signals to produce an alarm output if a match in the pattern of the input signal with one of the algorithms occurs. This system consequently eliminates the necessity for a large number of cumbersome and expensive tuned filters for its operation. The algorithms which are programmed into the microprocessor of Jensen each begin with a "start" frequency range and continously cycle to determine whether an input frequency fits within the start frequency range. The recycling time is relatively short, but it is necessary for the received siren pattern detected by the system to be at the "start" or beginning of a siren cycle to insure the proper sequential operation of the algorithm for determining whether a valid "siren" signal is present.
The system disclosed in the Koehler Patent is designed to detect a pair of spaced frequencies which are within the siren frequency range. Koehler relies upon a "wailing" siren signal which has a repetition frequency at a relatively low rate. The detected signals produce an output of the repetition frequency which then is used to trigger the alarm. Noise signals occuring at repetition rates other than the siren signal repetition frequency, even if they do include signals of the same frequencies as the two siren signal frequencies, will not have the required repetition rate. Consequently, such noise signals will not permit triggering of the alarm system. In an extremely noisy, heavy traffic environment, such as typically found in large cities, it may be possible to false trigger the Koehler system.
In addition, the system of Koehler assumes a relatively uniform repetition frequency of the siren. Modern emergency vehicles, however, do not always operate the siren in the same mode. When such a vehicle approaches a traffic intersection, the driver frequently switches the siren from a "wailing" operating mode to what is known as the "yelp" operating mode. The repetition frequency of a siren operating in the "yelp" mode is higher than the frequency of a "wailing" mode. Thus, if the Koehler system is adjusted to detect a "wailing" siren, it may not detect a "yelp" siren and vice-versa.
A different approach for tracking a "wailing" siren frequency is disclosed in the Stefanov Patent. In Stefanov, however, the "wailing" frequency rate is tracked by a voltage tunable filter. If a siren is present, this "wailing" signal occurs at a fixed rate, which generally is uniform for most emergency vehicle sirens. The outputs for the voltage tuneable filter controlled by the "wailing" signal then are passed through low and high frequency filters to control the operation of the alarm circuit. The voltage tuneable filter provides an output corresponding to the dominant frequency; and this output is converted to a voltage by means of a frequency-to-voltage convertor which then is fed back to the band-pass filter to shift the center frequency of that filter. Thus, the dominant frequency signal is tracked by the system. When this signal results from a "wailing" siren, the fed-back voltage from the convertor constitutes a slowly and continuously varying AC signal corresponding to the pitch variation of the siren. This signal is detected by a low-pass filter and a differentiating circuit to operate an alarm within an automobile. When the dominant frequency results from noise or steady-state sounds or varies at a rate other than the slowly varying AC signal, a different or random shifting of the center frequency of the band-pass filter occurs. Additional circuitry responsive to such random signal shifts then generates an inhibiting signal to prevent spurious operation of the alarm within the automobile. To be effective, this system requires several cycles of operation in order to produce an alarm output. Frequently, the number of cycles required may be such that the alarm within the automobile will not be triggered until the emergency vehicle is dangerously close. As with prior art patents which require a sequential or cascading detection of signals within the siren frequency range for proper operation, it is possible for Stefanov to fail to trigger an alarm even though an emergency vehicle siren is present, if that siren somehow is not operating properly in the expected manner, or if sound reflections from nearby buildings and the like distort some aspects of the siren signal.
The two Bernstein et al. Patents are directed to siren detection systems which are particularly suited for installation in an automobile and which have a high degree of selectivity and noise rejection capabilities. The Bernstein et al. systems employ a plurality of narrow band band-pass filters, the outputs of which are applied in parallel to an averaging circuit which produces a single output corresponding to the central one of any of the filters providing an output during a pre-established time interval. This output then is compared with the previous output (which indicated the previously selected band-pass frequency) to operate an up/down counter. Whenever successive counts of a pre-established number in the same direction occur, the output of the counter enables an alarm to indicate the presence of a siren operated by an emergency vehicle in the vicinity of the automobile. Since random noise signals do not produce the necessary successive output counts from the counter, the system starts a new count when successive outputs of the averaging circuit do not occur in the pre-established pattern. The Bernstein et al. systems operate to provide a relatively rapid determination of the presence or absence of a siren signal based on a relatively short time frame, but the systems of these patents require several band-pass filters which, as mentioned previously, results in additional expense and bulk for the system.
It is desirable to provide a siren detection alarm for use within an automobile which overcomes the disadvantages of the prior art, which is not subject to "false" alarms, and which rapidly and accurately provides an alarm indication without requiring a multiplicity of band-pass filters.